Tomography of solid materials

ABSTRACT

The invention provides a method for the identification of deposits of solid materials in process plants, the method comprising the creation of a theee-dimensional image by means of tomography, preferably passive tomography, most preferably gamma-ray tomography. Typically, the solid materials comprise gamma-ray emitting solid materials and the process plants comprise nuclear process plants. In general, the deposits of solid materials are deposited from liquid media, and the liquid media preferably comprise aqueous suspensions of these materials. The method is particularly applicable to highly active nuclear waste liquids in High Activity Storage Tank (HAST) applications, and offers significant advantages over the methods of the prior art, since it comprises a non-invasive technique which does not require the prior installation of a detector in the vessel from which data are to be obtained.

RELATED APPLICATION

This application is a national phase application of PCT InternationalApplication PCT/GB2004/002056, filed May 13, 2004, and published inEnglish on Nov. 25, 2004 as International Publication No. WO2004/102172, which claims priority from British Application No.0310998.0, filed May 14, 2003. These disclosures are hereby incorporatedby reference herein in their entireties.

This invention is concerned with a novel, non-invasive means for thedetection and three-dimensional imaging of quantities of solidsmaterials deposited from liquid media in industrial equipment,particularly storage vessels. More specifically, it relates to a novelapplication of high energy gamma-ray tomography to the identification ofthe location and geometry of accumulations of gamma-emitting solids fromnuclear process plants. Such accumulations are encountered in organicand aqueous suspensions, and in gaseous streams, most particularly inwaste treatment processes in the nuclear processing industry.

The storage and transfer of gamma-emitting radionuclides is common inthe nuclear processing industry. Consequently, the identification of thelocation and approximate geometry of accumulations of gamma-emittingsolids is of interest, to ensure operability of plant and safe storageof highly emitting materials. This is particularly true in the case ofso called “High Activity Storage Tank” (HAST) applications, whereinsignificant volumes of highly active liquors from nuclear fuelreprocessing, which have been concentrated up to 100-fold byevaporation, are stored in cooled double-walled tanks, generallyconstructed from stainless steel.

The known technology in this field has generally relied on the proveneffectiveness of the installed agitation equipment, and on temperaturedetection as the monitoring means. Thus, it has been necessary toinstall temperature detectors in the equipment, the detectors having tobe located in the same positions as the solid deposits, and theireffective operation relying on the presence of sufficient depositedmaterial to promote a temperature rise of the material above that of thebulk liquor which is sufficiently significant to be detectable.

Naturally, there are disadvantages with this approach. In the firstinstance, it is necessary for the relevant temperature detection meansto be installed in the equipment during construction. Furthermore, it isnecessary that the equipment designer should be able to accuratelypredict the correct location for the detector within the equipment;clearly, the detector must be located at a position, or positions, wheresolids deposits will occur if it is to be able to perform its functioneffectively, but it may not always be possible to make such predictionswith complete accuracy at the outset. Consequently, it is quite likelythat, in use, this means of detection may completely fail to identifysome deposits, if these occur in locations which are not provided with adetector. In addition, of course, it has to be borne in mind that sincethe method relies on temperature increases as the means of detection,there is the possibility that temperatures may reach undesirably highlevels before operators are alerted to the presence of accumulatedsolids deposits.

Clearly, therefore, there is a requirement for a more efficient, saferand preferably non-invasive means to allow for the ready detection ofsolids deposits in such circumstances, and it is this objective whichthe present invention seeks to address. The invention thus overcomes thedeficiencies of the prior art, and facilitates the convenientidentification of the location and geometry of accumulations ofgamma-emitting solids, thereby significantly enhancing the prospects forthe safe storage of highly gamma-emitting materials.

The present invention relies on a novel and inventive application of thetechnique of tomography, more specifically gamma-ray tomography.Tomography first found significant practical application in the medicalfield, where X-ray tomography allowed for non-intrusive diagnoses to beperformed by creating cross-sectional images from scanned data obtainedfrom the human body. The technique of tomography relies on compiling aseries of data measurements from an array of sensors and then processingthe data using an appropriate image reconstruction algorithm run onsuitable computer hardware in order to generate a suitable image on ascreen.

Following the initial research in the field of medical imaging, a numberof applications of tomographic imaging of process equipment weredescribed, but generally these involved using ionising radiation fromX-ray or isotope sources, and were not found to be satisfactory for themajority of process applications on a routine basis because of the highcost involved and safety constraints. Most of the radiation-basedmethods used long exposure times which meant that dynamic measurementsof the real time behaviour of process systems were not feasible. Suchmethods furnished examples of active tomographic techniques, wherein anexternal signal is provided and the response of a medium to the signalis measured.

However, as well as developing and refining these active tomographicmethods, later work also focused on an alternative tomographic approach,wherein the variation of some inherent property of a medium wasassessed. Several methods of this type—known as passive processtomography systems—were developed, and may rely on, for example, themeasurement of electrical parameters, such as capacitance, impedance andresistance, by means of an array of sensors placed around the peripheryof a process vessel, to create an image of the concentration andmovement of components inside, both cost-effectively and in real-time.Measurements were reconstructed to form 2- or 3-dimensional images,providing information to monitor processes and improve yields, quality,efficiency and overall control. Electrical impedance tomography alsofound application as a safe, low-cost method for imaging the human body.

Process tomography has now been applied to many types of processes andunit operations. Depending on the sensing mechanism used, it isnon-invasive, inert and non-ionising, and is therefore applicable in theprocessing of raw materials, in large-scale and intermediate chemicalproduction, and in the food and biotechnology areas. A number oftomographic processes have become available for studying complexmultiphase phenomena. These include, for example, infrared and opticaltomographic systems, positron emission tomography (PET), magneticresonance imaging (MRI), and sonic or ultrasonic tomographic systems,and various of these tomograpic systems have been used to imagemultiphase processes such as fluidisation, pneumatic conveying,liquid/liquid and gas/liquid mixing and solid/liquid separationoccurring in pipelines, stirred reactors, fluidised beds, mixers andseparators.

The use of the technique of gamma-ray tomography for the study of liquidholdup distribution in large packed columns has been disclosed by F Yin,A Afacan, K Nandakumar and K T Chuang, Chem. Eng Process., 41 (2002),473-483. The method involved the measurement of two liquid flow rates bymeans of horizontal scans, taken at two vertical positions. Yin et alalso report the successful application of this technique in thetroubleshooting of distillation columns, the measurement of gas holdupprofiles in bubble columns and the measurement of local porositydistribution in packed columns.

A more interesting application of gamma-ray tomography is in the assayof drums containing radioactive waste. The Waste Inspection Tomographyfor Non-Destructive Assay (WIT-NDA) system developed by LawrenceLivermore National Laboratory and Bio-Imaging Research combines activeand passive computed tomography and nuclear spectroscopy to accuratelyquantify all detectable gamma-rays emitted from waste containers. Thistechnique, disclosed in Nuclear Engineering International, 46(59), 18,has been used for the non-destructive examination and assay ofradioactive waste since 1999.

However, whilst the WIT-NDA system has been found to be extremely usefulfor the examination of nuclear waste materials stored in drums, thesystem only has limited applicability, since such waste is inevitably insolid form, for example encapsulated in concrete or vitrified withglass-forming materials. Clearly, in such circumstances, the wastematerials are immobilised within a solid matrix and the situation isvery different to that addressed by the present inventors, who havesought to identify solid deposits and accumulations which occur in fluidmedia, specifically liquid suspensions of the said materials, whereinthere is a degree of mobility of all the components.

Particular difficulties may be encountered in situations where bulkfluid media, particularly bulk liquid media, are being handled inlarge-scale storage tanks, since access for measurements may be severelylimited due to a high-radiation environment existing in the vicinity ofthe tanks. In such circumstances, it is important to know whether or notthe contents of the tank are well mixed, which is the desirablesituation, or if some of the solid has come out of suspension anddeposited at some location within the tank.

Surprisingly, the present inventors have now found that it is possibleto successfully apply the technique of tomography to the identificationof accumulations of solid materials in nuclear process plants. Since thepresent application comprises a passive tomographic technique, whereinthe shape, signal strength and distribution of the source material isunknown, the problems of implementation of such a technique are thatmuch greater, but the inventors have found that it is possible toachieve accurate images of accumulations of material in suchcircumstances by means of this technique.

Additionally, a method has been found which facilitates the productionof two images, one of which shows the distribution of the sources of theemissions, whilst the other shows the structure of the attenuatingmedium between the sources and the detectors.

Various aspects of the present invention will now be described in moredetail with respect to other embodiments described herein. It should beappreciated that the invention can be embodied in different forms andshould not be construed as limited to the embodiments set forth herein.Rather, these embodiments are provided so that this disclosure will bethorough and complete, and will fully convey the scope of the inventionto those skilled in the art.

The terminology used in the description of the invention herein is forthe purpose of describing particular embodiments only and is notintended to be limiting of the invention. As used in the description ofthe invention and the appended claims, the singular forms “a,” “an”and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. Also, as used herein, “and/or”refers to and encompasses any and all possible combinations of one ormore of the associated listed items.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. All publications, U.S. patentapplications, U.S. patents and other references cited herein areincorporated by reference in their entireties with respect to the textreferenced by the citation.

Thus, according to the present invention, there is provided a method forthe identification of deposits of solid materials in process plants,said method comprising the creation of a three-dimensional image bymeans of tomography. Specifically, the method provided by the presentinvention comprises passive tomography, wherein there is no requirementfor the application of an external field, and the technique relies onthe emission of radiation by the system under investigation. The emittedradiation may be radioactive in nature; in the preferred situation, saidradiation comprises gamma-radiation, and the preferred monitoringtechnique comprises gamma-ray tomography.

More specifically, said solid materials comprise gamma-ray emittingsolid materials and said process plants comprise nuclear process plants.Said deposits of solid materials comprise deposits which are depositedfrom gaseous media or, more particularly, from liquid media. Typically,said liquid media comprise liquid suspensions of said materials, forexample, suspensions of said materials in organic solvents but, morepreferably, aqueous suspensions of said materials. Most preferably saidliquid suspensions of said materials comprise highly active nuclearwaste liquids.

Said deposits typically comprise accumulations of said solid materialswhich occur in, for example, storage tanks, particularly large scalestorage tanks. Alternatively, it is envisaged that said deposits couldoccur in pipework used to transfer the liquid media between vessels. Themethod of the present invention, however, finds particular applicabilityin the monitoring of radioactive waste in “High Activity Storage Tank”(HAST) applications.

The identification of said solid deposits typically involves theidentification of the location of said deposits and the identificationof the geometry of said deposits. Said identifications are facilitatedby the creation of three-dimensional images of the deposits by means ofgamma-ray tomography. Thus, by means of image reconstruction,gamma-emission tomography becomes a powerful tool for industrieshandling radioactive materials. Optimum results are achieved withmaterials stored in storage tanks having a diameter of around two metresor less, in which case 100% imaging may be achieved.

The method of the present invention relies on the measurement, by meansof an array of sensors, of emissions, preferably gamma-ray emissions,from process equipment, which typically comprises a storage tank, andthe subsequent manipulation of the obtained data by means of a suitablecomputer algorithm in order to generate a three-dimensional image. Themethod involves the computation of a solution to the Boltzmann equationfor gamma-ray transport. This exercise is in the class of inverseproblems, whereby the distribution of the source terms is derived on thebasis of measurements made at the boundary of the process plant.

As previously discussed, the method of the present invention findsparticular application in the identification of solid accumulations instorage tanks, but is also valuable in detecting blockages which mayoccur in pipework, and for the detection of leaks which might beoccurring from pipework, storage tanks, or any other vessels.

It also finds application in monitoring the efficiency of stirring orother forms of agitation which are being used in vessels, since anyunwanted deposition of material can be readily identified.

In addition, as observed earlier, the method can be adapted for theproduction of two images, the first of these showing the distribution ofthe sources of the gamma-ray emissions, whilst the other depicts thestructure of the attenuating medium between the sources and thedetectors.

The technique offers significant advantages over the methods of theprior art which were used for the monitoring of such solid deposits,most particularly in view of the fact that it comprises a non-invasivetechnique which does not require the prior installation of a detector inthe vessel from which data are to be obtained. In addition, it allowsfor the investigation of a system with no prior knowledge of itscontents, unlike the Waste Inspection Tomography for Non-DestructiveAssay (WIT-NDA) system of Lawrence Livermore National Laboratory andBio-Imaging Research, which has previously been discussed.

The method of the present invention is also distinguished over variousmethods of the prior art in that it relies only on the emission, ratherthan the transmission, of radiation; furthermore, unlike the knowntechniques which provide two-dimensional, images, the present techniquefacilitates the production of images in three dimensions.

1. A method for the identification of deposits of solid materials inprocess plants, said method comprising the creation of athree-dimensional image by means of tomography.
 2. A method as claimedin claim 1 wherein said tomography comprises passive tomography.
 3. Amethod as claimed in claim 1 wherein said tomography comprises gamma-raytomography.
 4. A method as claimed in claim 3 wherein said solidmaterials comprise gamma-ray emitting solid materials and said processplants comprise nuclear process plants.
 5. A method as claimed in claim1 wherein said deposits of solid materials comprise deposits which aredeposited from gaseous media.
 6. A method as claimed in claim 1 whereinsaid deposits of solid materials comprise deposits which are depositedfrom liquid media.
 7. A method as claimed in claim 6 wherein said liquidmedia comprise liquid suspensions of said solid materials.
 8. A methodas claimed in claim 7 wherein said liquid suspensions of said solidmaterials comprise suspensions of said solid materials in organicsolvents.
 9. A method as claimed in claim 7 wherein said liquidsuspensions of said solid materials comprise aqueous suspensions of saidsolid materials.
 10. A method as claimed in claim 9 wherein said aqueoussuspensions of said solid materials comprise highly active nuclear wasteliquids.
 11. A method as claimed in claim 1 wherein said depositscomprise accumulations of said solid materials occurring in storagetanks.
 12. A method as claimed in claim 11 wherein said storage tankscomprise large scale storage tanks.
 13. A method as claimed in claim 11wherein said storage tanks have a diameter of around two meters or less.14. A method as claimed in claim (1) wherein said deposits compriseaccumulations of said solid materials in pipework used to transferliquid media between vessels.
 15. A method as claimed in claim 1 whereinthe identification of said solid deposits comprises the identificationof the location of said deposits and the identification of the geometryof said deposits.
 16. A method as claimed in claim 1 which comprises themeasurement of emissions from process equipment and the manipulation ofthe obtained data in order to generate a three-dimensional image.
 17. Amethod as claimed in claim 16 wherein said measurement of emissions isachieved by means of an array of sensors.
 18. A method as claimed inclaim 16 wherein said measurement of emissions comprises measurement ofgamma-ray emissions.
 19. A method as claimed in claim 16 wherein saidmanipulation of the obtained data in order to generate athree-dimensional image comprises manipulation by means of a computeralgorithm.
 20. A method as claimed in claim 16 wherein said manipulationof the obtained data involves the computation of a solution to theBoltzmann equation for gamma-ray transport.
 21. A method for theidentification of deposits of solid materials in process plants, saidmethod comprising the creation of a three-dimensional image by means oftomography wherein said deposits comprise accumulations of said solidmaterials occurring in storage tanks and said storage tanks are largescale storage tanks comprising High Activity Storage Tanks.
 22. A methodfor the identification of deposits of solid materials in process plants,said method comprising the creation of a three-dimensional image bymeans of tomography, wherein said tomography comprises gamma-raytomography which is adapted for the production of two images, the firstof these showing the distribution of the sources of gamma-ray emissionsand the other depicting the structure of the attenuating medium betweenthe sources and the detectors.